The present invention relates to magnetic bubble memories, and more particularly to methods of increasing the information bit storage capacity for given chip sizes.
A conventional field-access magnetic bubble memory includes a plurality of permalloy propagation elements which overlie a film of magnetic garnet. The permalloy elements are typically configured in the shape of assymmetric chevrons or C-bars which are positioned relative to one another to define a plurality of paths. These paths are adapted for having magnetic bubble domains propagated along the same under the influence of Z-bias magnetic field and an in-plane rotating XY magnetic drive field. In most existing and proposed bubble memory chips, the magnetic bubbles travel along major and minor loops. Usually, a surplus of minor loops are included in the on-chip memory architecture to provide built-in redundancy that is vital if decent chip yields are to be obtained with high storage capacity chips. The loops can be arranged in various ways, but their organization is typically dictated by the active components of the chip--the bubble generators, replicators, detectors, and gates for transferring bubbles between the major and minor loops. These components in turn will be affected by increasing the information bit storage capacity of the chip.
There is an ongoing effort to increase the information bit storage capacity for given bubble memory chips sizes. To a limited extent, this will reduce the overall physical size of a memory system incorporating a plurality of bubble memory chips. However, the primary motivating reason for this ongoing effort is to achieve a reduction in the memory system price per bit. By way of example, if the size of a one megabit bubble memory chip can be reduced from 400 mils on a side to 350 mils on a side, then a significantly larger number of individual bubble memory chips can be simultaneously fabricated on a single wafer. Generally, the price per storage bit decreases as the number of memory chips of a given storage capacity produced from a given wafer size increases.
In most conventional field-access bubble memories, the active components and the paths leading to and from the same are located in a plurality of control function areas which are typically the peripheral regions of the chip. The propagation paths which comprise the plurality of data storage loops are usually located in a medial or central portion of the chip. Usually a major portion of the chip area is devoted to the data storage loops. If any significant increase in the information bit storage capacity for a given chip size is to be achieved, the bit density in the storage area must be increased.
A number of problems arise when seeking to increase the bit density in the storage area. First of all, in order to form propagation paths, it is necessary to form a gap between adjacent permalloy propagation elements. The gap size is typically only one half to two thirds of the bubble diameter (d). Presently, current materials growth technology and photolithographic resolution limits establish a lower limit for d of approximately 1.5 to 2 microns. Furthermore, magnetic bubbles must be spaced apart by about 4d, or 6 to 8 times the gap. This spacing is necessary to avoid adjacent bit interaction. If the bubbles are too close together, they tend to self bias each other and collapse.
A conventional design rule for field-access bubble memories employing assymmetric chevron or C-bar permalloy elements is that the period (p) should equal approximately 4.25 times d. The period is the length of the element from tip to tip. A theoretical lower limit for the storage area required for one bit, herein referred to as the storage cell, is approximately p.sup.2. These design rules yield good bubble propagation margins in terms of wide Z-bias ranges and low magnitudes for the required drive fields.
Heretofore, Texas Instruments, Inc. has reduced the period in the storage area relative to the control function areas. However, this has resulted in an undesirable net margin reduction. It is believed that a similar approach has been taken in one design of International Business Machines Corporation. In that design, along with the period difference the garnet film was made approximately 2 microns in the control function areas and approximately 1.5 microns in the storage area. Varying the thickness of the garnet film in this way offsets the net margin reduction that would otherwise occur because of the reduced period in the storage area. However, it is believed that the garnet film is thinned in the storage area by ion milling and this presents further problems in terms of uniform bubble integrity.